1. Field of the Invention
The present invention relates to a line printer and, more particularly, to a belt synchronous check system for a line printer.
2. Description of the Prior Art
In a prior art line printer using a type-belt changing system or a train cartridge changing system, standard type-belts may be replaced by the operator. For example, a standard 64-character type-belt comprises 6 sets of 64 characters, and a standard 96-character type-belt comprises 4 sets of 96 characters. That is, each standard type-belt comprises a total of 384 characters. In such a standard type-belt, a home position (HP) mark is provided for each character set, and a clock (CL) mark is provided for each character.
Usually, the line printer comprises an interface for responding to a center machine, a print data buffer (PDB) for receiving print data from the center machine, a belt image memory (BIM) for storing standard belt character codes, detectors for the HP and CL marks on the running type-belt, a belt synchronous circuit, a hammer mechanism, and the like.
The line printer operates as follows. If the printer is in a normal state, the printer enters into a data transfer mode. In this mode, print data is transferred from the center machine through the interface to the PDB. The data transfer is terminated when the PDB receives a definite number of data, 136 digits of data, or when the center machine stops sending the print data. After this data transfer mode is completed, the printer enters into a print mode. In the print mode, the BIM is scanned by reference to CL signals and HP signals generated from the type-belt mechanism. At this stage, the character code of the BIM corresponding to the character in front of the hammer is compared with the character at each position, and as a result, when a matching character is found, the hammer is activated so as to print the character. This processing is carried out for all the positions.
The above-mentioned operation is usually carried out after the characters on the type-belt have been synchronized with the content of the BIM. However, a mismatching will occur in the synchronization between the type-belt and the BIM if any of the following phenomena develop:
1. The number of characters per set is not equal to a predetermined value, such as 384, when an HP mark is detected.
2. An HP mark is not detected when the number of characters reaches the predetermined value, such as 384.
In order to carry out a belt synchronous check, a decoder is provided in the belt synchronous circuit for checking whether or not the number of characters reaches a predetermined value. When the number of characters reaches the predetermined value, the decoder generates a second home position (HP-D) signal which is compared with a first HP signal obtained from an HP mark. If a mismatching is generated between the two signals, the printer indicates that there is a possibility of character errors, missed characters, or the like in the preceding two or three lines.
In the above-mentioned prior art, however, since such a predetermined value which determines the number of standard type-belts, is set by, the number of wire connections of the input terminals of the decoder, therefore, the number of predetermined values, that is, the number of standard type-belts available is limited.